Vincent Tuck is a private detective based in Wapping, London. According to the last letter of Doctor Lionel Trollope, Tuck was hired by Grahame Roby to investigate his brother Alexander Roby's associates.

Andrew Saunders and Nathaniel Browne called on Tuck. who became upset at the mention of Alexander Roby, but eventually agreed to tell the investigators what he knew of the case.

In short, Tuck spent three weeks following Roby in late 1925, and saw that he spent most of his evenings with Lawrence Bacon, Malcolm Quarrie and Montague Edwards, though Tuck has no real idea what they were doing. He also provided background information on the three men.

After several drinks in a nearby pub, Tuck convinced himself to give Saunders a typed report in which he describes Bacon apparently using black magic to kill a vagrant.

Tuck clearly has a drink problem, possibly worsened by what he saw. He has asked Saunders and Browne not to come back.